1. Field of the Invention
The present invention relates to an apparatus and method for improving voice clarity. More specifically, the present invention relates to an apparatus and method for improving voice clarity to provide improved intelligibility of voice navigation even in a noisy environment while considering a maximum allowable volume level safe for protection against hearing loss.
2. Description of the Related Art
Commercially available vehicle navigation systems and hands-free telephone systems may include a voice output device that employs an apparatus for improving voice clarity using loudness compensation. Such voice-clarity improving apparatuses may provide intelligible voice navigation or guidance instruction in noisy environments that include the sound of audio playback in a vehicle, the sound of the vehicle engine, and the sound of traveling, where it is difficult to hear voice navigation or guidance instruction. The volume level can be appropriately adjusted depending upon the level of noise.
FIG. 7 is a block diagram of a voice-clarity improving apparatus of the related art (see, for example, Japanese Unexamined Patent Application Publication No. 11-166835). The voice-clarity improving apparatus 1 includes a volume adjusting unit 11 for manually adjusting the volume level of a voice signal, and a gain corrector 12 including an attenuator and a variable gain unit for controlling a gain G of the voice signal so as to provide a clear voice. The voice-clarity improving apparatus 1 further includes an amplifier 13 for amplifying the voice signal, a speaker 14 for radiating a voice into the vehicle cabin, a microphone 15 for detecting a combination of noise and voice, a signal separating unit 16 for separating and outputting a voice signal and a noise signal, a loudness-compensation-based gain determining unit 17 for determining a gain Gopt of a voice signal using loudness compensation so that a clear voice can be reproduced, a white noise source 18 for use in determining transmission characteristics of an acoustic communication system including the speaker 14 and the microphone 15, and a switch 19.
An audio sound playback unit 2 is separate from the voice-clarity improving apparatus 1. The audio sound playback unit 2 includes an audio source 21, such as a compact disc (CD) player, a Mini-Disk player, or a tuner, an amplifier 22, and a speaker 23. Audio playback sound is output from the speaker 23.
In the voice-clarity improving apparatus 1, the volume adjusting unit 11 adjusts the volume level of a voice-navigation signal input from a navigation apparatus. The gain corrector 12 multiplies the gain Gopt determined by the loudness-compensation-based gain determining unit 17 to the volume-adjusted voice-navigation signal. The amplifier 13 amplifies the voice-navigation signal output from the gain corrector 12. The speaker 14 radiates the voice navigation input from the amplifier 13 into the vehicle cabin. The microphone 15 receives a combination of the audio playback sound output from the speaker 23 of the audio sound playback unit 2, environmental noise, and voice navigation, and inputs the combined sound to the signal separating unit 16.
The signal separating unit 16 separates a voice signal and a noise signal that is a composite signal having an audio playback sound signal and a noise signal. The signal separating unit 16 includes an identification filter 16a for simulating an impulse response of the acoustic communication system including the speaker 14 and the microphone 15, a calculating unit 16b, and an adaptive control device 16c. The adaptive control device 16c includes a least-mean-square (LMS) adaptive controller 31 and an adaptive filter 32. The adaptive controller 31 performs adaptive control to identify an impulse response of the acoustic communication system including the speaker 14 and the microphone 15, and passes the identified impulse response to the adaptive filter 32. The transmission characteristics of the acoustic communication system determined by adaptive control are copied to the identification filter 16a. 
The loudness-compensation-based gain determining unit 17 determines the optimum gain Gopt to be added to the voice-navigation signal based on the voice-navigation signal output from the identification filter 16a and the noise signal (i.e., the audio playback sound signal and the noise signal) outputted from the calculating unit 16b, and inputs the gain Gopt to the gain corrector 12.
The principle of determining the gain using loudness compensation will now be described.
FIG. 8 is a loudness curve indicating the relationship between the physical sound pressure level and the magnitude of physiological sensation produced by a sound (hereinafter referred to as “loudness”). In the loudness curve shown in FIG. 8, the x axis indicates the sound pressure level (SPL) in decibels (dB), and the y axis indicates the loudness in sones. In FIG. 8, a loudness curve (a) is exhibited in a quiet environment, and a loudness curve (b) is exhibited in a noisy environment where the minimum value of human audible sensation produced by a sound is about 35 dB higher. The loudness curve (b) varies depending upon the noise.
The loudness curve indicates that a sound having the same loudness on the y axis is perceived as the same sound. In FIG. 8, a sound with a loudness of 0.1 sone is perceived only at an SPL of 12 dB in the quiet environment indicated by the loudness curve (a), and is perceived at an SPL of 37 dB in the noisy environment indicated by the loudness curve (b). In other words, in order to perceive a sound having the same magnitude as that perceived at an SPL of 12 dB in the quiet environment indicated by the loudness curve (a), the sound must have an SPL of 37 dB in the noisy environment indicated by the loudness curve (b). In this case, an additional gain of 25 dB is required. A sound having one sone is perceivable at an SPL of 42 dB in the quiet environment indicated by the loudness curve (a), and is perceivable at an SPL of 49 dB in the noisy environment indicated by the loudness curve (b). In this case, an additional gain of 7 dB is required. Gain is therefore changed depending upon the level of the voice-navigation signal in noisy environments.
FIG. 9 is a graph showing the relationship between the voice signal level and the gain in a noisy environment. In FIG. 9, the x axis indicates the SPL (corresponding to the level of the voice signal) in a quiet environment, and the y axis indicates the gain required to perceive a sound in the noisy environment indicated by the loudness curve (b) shown in FIG. 8 as a sound having the same magnitude as that in a quiet environment. As indicated by the loudness curve (b) shown in FIG. 8, a noisy environment where the minimum value of human audible sensation produced by a sound is about 35 dB higher has been described by way of example.
As shown in FIG. 10, the loudness curve also changes as the noise level changes, and therefore the gain-to-signal characteristic also changes depending upon the noisy environment, as shown in FIG. 11. In FIG. 10, a loudness curve (a) is an optimum curve in which the loudness f is given by the following equation:f=KIwhere I denotes the intensity of sound. A loudness curve (b) is exhibited in an environment with physiological noise, such as blood flowing sounds, without external noise (this environment corresponds to the quiet environment indicated by the loudness curve (a) shown in FIG. 8). A loudness curve (c) is exhibited in a noisy environment where the minimum value of audible sensation produced by a sound is 15 dB higher. A loudness curve (d) is exhibited in a noisy environment where the minimum value is 35 dB higher, and a loudness curve (e) is exhibited in a noisy environment where the minimum value is 55 dB higher.
The loudness-compensation-based gain determining unit 17 stores gain-to-signal characteristics (see FIG. 11) for various noise levels in an internal memory in advance, and selects a gain-to-signal characteristic for an actual noise level in a vehicle cabin. Then, the loudness-compensation-based gain determining unit 17 determines the gain Gopt optimum for the level of the voice-navigation signal based on the selected gain-to-signal characteristic, and inputs the optimum gain Gopt to the gain corrector 12. The gain corrector 12 multiples the optimum gain Gopt to the voice-navigation signal. Therefore, even in a noisy environment, a user can perceive a sound having the equivalent level to that in a quiet environment.
In the voice-clarity improving apparatus 1 shown in FIG. 7, before starting normal operation, it is necessary to identify an impulse response of the acoustic communication propagation system including the speaker 14 and the microphone 15 and to set the identified impulse response in the identification filter 16a. When the impulse response is identified, the voice signal and the audio playback sound signal are set to 0. In this state, the white noise source 18 is activated, and the switch 19 is turned on. Then, white noise is radiated into an acoustic space in the vehicle cabin from the speaker 14 through the amplifier 13, and is detected by the microphone 15. The white noise is also input to the adaptive control device 16c, and is then filtered by the adaptive filter 32.
The calculating unit 16b subtracts the output signal of the adaptive filter 32 from the detection signal of the microphone 15, and feeds back the difference to the adaptive control device 16c as an error signal. The adaptive controller 31 performs adaptive control using an LMS algorithm so that the power of the error signal can be minimized to determine a coefficient of the adaptive filter (finite impulse response (FIR) filter) 32. The adaptive control described above is repeatedly performed until the output of the adaptive filter 32 is equal to the output of the microphone 15, and therefore the impulse response of the acoustic communication system is set in the adaptive filter 32. More specifically, the impulse response characteristics, i.e., the transmission characteristics, from an input from the amplifier 13 to the microphone 15 are set in the adaptive filter 32.
After the impulse response of the acoustic communication system is identified, the coefficient of the adaptive filter 32 is copied to the identification filter 16a. Then, the switch 10 is turned off to enable normal operation.
In the normal operation, the identification filter 16a applies the impulse response characteristic of the acoustic communication system to the voice-navigation signal input via the volume adjusting unit 11 and the gain corrector 12 to generate a pseudo audio signal at the observation point (i.e., at the position of the microphone 15), and inputs the audio signal to the loudness-compensation-based gain determining unit 17. The calculating unit 16b subtracts the voice-navigation signal from the combined-sound signal at the observation point, which is output from the microphone 15, to generate a noise signal (including audio playback sound and external noise), and inputs the noise signal to the loudness-compensation-based gain determining unit 17.
The loudness-compensation-based gain determining unit 17 selects a gain-to-signal characteristic for the noise level to determine the gain Gopt optimum for the level of the voice-navigation signal based on the selected gain-to-signal characteristic, and inputs the optimum gain Gopt to the gain corrector 12. That is, the loudness-compensation-based gain determining unit 17 inputs the optimum gain Gopt to the gain corrector 12 in order to make the loudness of the voice-navigation signal in a noisy environment equal to the loudness of the voice-navigation signal in a quiet environment. The gain corrector 12 multiplies the optimum gain Gopt to the input voice-navigation signal. The above-described control is repeatedly performed, so that a user can receive the voice navigation even in a noisy environment in the same way as in a noiseless environment.
With the ability to reduce noise in vehicle cabins and the prevalence of in-vehicle devices, such as vehicle audio devices, audio-visual devices, and navigation systems (such devices are hereinafter referred to as “AV devices”), users often listen to AV sources (including music, TV sound, and voice navigation) in vehicle cabins. The amplification level for the AV sources can be increased to any desired level. In vehicle cabins, which are convenient private spaces, users often listen to the AV sources at a higher volume level than usual (e.g., at home).
In a noisy environment where audio is played back, it is typically necessary to correct the voice-navigation signal so as to have a level higher than noise to receive voice navigation. However, it is undesirable for the voice-clarity improving apparatus to increase the gain of the voice signal for any noise level without limit. Sound with too high a volume level can cause hearing loss.
Generally, passengers who are in a loud AV source environment for a long time can often suffer from hearing loss. Although depending upon the individual, it is said that people exposed to a daily noise level of 80 dB or higher will possibly suffer from hearing impairment. In France, a sound limit of 100 dBA was set by law, and a maximum output level lower than 100 dBA for stereo headphone volume has been regulated and enforced since 1998.
Therefore, there is a demand for a voice-clarity improving apparatus with a maximum gain of voice navigation safe for protection against hearing loss. Mere control of the maximum gain does not allow a user to sufficiently hear a voice in a noisy environment where audio is played back.